1. Field of the Invention
The present invention relates to the field of rectifier circuits and more particularly, to synchronous rectifier circuits.
2. Prior Art
Semiconductor rectifiers have long been used in power supplies to convert an AC input voltage to a DC output voltage. As used herein, the term "rectifier" means a device whose internal structure conducts current in one polarity, but blocks current flow in the opposing polarity. Rectifiers include diodes and SCRs. Such DC power supplies may employ either half-wave rectification in which a single rectifier is used or full-wave rectification in which a transformer, a bridge circuit and two discrete rectifiers are used in order to rectify both halves of an AC input voltage. Such DC power supplies, in which the power supply DC output voltage is large compared to the ON-state voltage drop of the rectifier, provide fairly efficient conversion of an AC input voltage to a DC output voltage. However, as the power supply DC output voltage becomes a smaller multiple of the rectifier forward voltage drop, the power conversion efficiency of the power supply decreases because of an increase in the percentage of the AC input power which is dissipated in the rectifiers.
One way of minimizing the power loss in the rectifying devices of a DC power supply is to use synchronous rectification rather than discrete rectifiers to convert the AC input voltage to a DC output voltage In a synchronous rectifier, the rectification is performed by semiconductor switching devices operating under the control of a synchronized timing circuit. The switching devices in a synchronous rectifier circuit are capable of conducting current in both directions and may be normally off until turned on by a control signal, or normally on until turned off by a control signal or may require a control signal for both turn on and turn off.
The term "rectifying device" as used herein refers to a device which rectifies the AC current/voltage therethrough, independent of how it achieves that function. Thus, the term "rectifying device" includes the switching devices in a synchronous rectifier circuit as well as rectifiers per se. In a synchronous rectifier circuit, a control signal is provided to the switching device which causes it to be conductive when the voltage applied across it is of one polarity and to be non-conductive when the voltage applied across it is zero or of the opposite polarity. Where the rectifying device is a field effect transistor (FET) or another device whose forward voltage drop is less than that of a rectifier (diode), the portion of the applied power dissipated in the rectifying device of a synchronous rectifier circuit is reduced as compared to that dissipated in the rectifier for the same DC output voltage. Bipolar transistors, as well as FETs, have been used as the rectifying devices in synchronous rectifier circuits. In order to maximize the efficiency and density of such synchronous rectifiers, the rectifying devices used therein should have as small an inherent ON-state voltage drop as possible and should have as low an ON-resistance per unit area as possible.
Prior art devices which have been used as rectifying devices in synchronous rectification either have an internal structure which limits the maximum frequency at which they can rectify high amplitude currents/voltages and/or include parasitic devices which prevent the synchronous rectifier from operating in an ideal manner. We have found that the maximum operating frequency of a FET synchronous rectifier circuit is limited by parasitic diodes and a parasitic bipolar transistor which are inherent in the FET. In an FET based synchronous rectifier circuit one of the parasitic diodes within the FET becomes conductive at some stages in the operating cycle of a synchronous rectifier and cause switching noise which is present in the DC output voltage and may also interfere with the synchronous control system. The parasitic bipolar transistor in an FET can cause destruction of the FET under some circuit conditions.
Accordingly, it is an object of the present invention to provide a synchronous rectifier circuit which is substantially free of parasitic bipolar transistor and diode devices.
It is another object to provide a synchronous rectifier system in which lossy parasitic diodes are prevented from becoming conductive.
It is a further object to provide a synchronous rectifier system having fast diodes in place of slow diodes.